Digital printing presses and other digital printing devices (hereinafter “printing devices”) may incorporate an electro-optical instrument, typically a reflection spectrophotometer, for automatically controlling print attributes. FIG. 1 illustrates a conventional arrangement of a reflection spectrophotometer 15 within a printing device 10. As shown, the spectrophotometer 15 may be oriented above a media guide 20 and include an illumination source 25 (e.g., an LED or lamp) for illuminating print media 30 (e.g., paper) via a first guide aperture 35 as the media 30 is passed through the guide 20. Although depicted as a slotted guide, the guide 20 may alternatively be a roller or other device for suitably directing the media 30. Light reflected from the media 30 is received by a sensor module 40 within the spectrophotometer 15 via an aperture 45. Although not shown for the sake of clarity, the sensor module 40 may include optics (e.g., lens, mirrors, etc.), light detectors (e.g., CCD sensors), and various electronics configured for processing the reflected light and generating spectral data therefrom. The spectral data may be communicated to a print engine (not shown) within the printing device 10 and used, for example, to control colors printed by the printing device 10 in accordance with a color desired standard. In order to ensure that the spectral data accurately represents the color characteristics of the media 30, the orientation of the spectrophotometer 15 is required to be such that its “read plane” (i.e., a plane parallel to the illuminating/detecting face of the spectrophotometer 15 corresponding to optimal illumination and light detection) precisely coincides with the upper surfaces of the media 30. Typically, the surfaces of the media 30 must be within several thousandths of an inch of a known read plane or at a known offset in order to obtain suitably accurate spectral data. Repeatability of such tolerances may be maintained, for example, by providing a reference surface (not shown) that contacts a portion of the illuminating/detecting face of the spectrophotometer 15 or reference features.
It is necessary to periodically calibrate the spectrophotometer 15 using a calibration reference. Typically, the calibration reference is matched to the spectrophotometer 15 and comprises a white source (e.g., a white ceramic disc) having a color characteristic traceable to a suitable color standard, such as that established by the National Institute of Standards and Technology. Non-white (e.g., red, green, and/or blue) calibration references may also be used. During calibration, spectral data generated by the spectrophotometer 15 using the calibration reference is compared to spectral data corresponding to the calibration reference that has been previously stored within the spectrophotometer 15. Based upon this comparison, a color transform curve for suitably compensating spectral data of subsequent measurements may be generated using known methods.
For the printing device 10 of FIG. 1, automatic calibration of the spectrophotometer 15 may be problematic due to the orientation of the spectrophotometer 15 relative to the guide 20. In particular, the structure of the guide 20 generally precludes physical placement of the calibration reference at the read plane, particularly in cases where the guide 20 is a roller.
FIG. 2 illustrates an alternative placement of a calibration reference 50 as is known in the art. As shown, the calibration reference 50 is placed below a second guide aperture 55 aligned with first guide aperture 35 such that that calibration reference 50 is illuminated through the guide 20. This arrangement may not be satisfactory, however, as placement of the calibration reference 50 outside of the read plane may degrade the accuracy of the resulting spectral data, thus degrading the calibration accuracy.
FIGS. 3a-3c illustrate sequential operation of an alternative arrangement known in the art for automatically calibrating the spectrophotometer 15. In FIG. 3a, a top view of the spectrophotometer 15 and guide 20 in the normal operating position is shown. The calibration reference 50 is positioned adjacent to a side of the guide 20. During calibration, the spectrophotometer 15 is taken offline and mechanically translated such that its read plane coincides with the upper surface of the calibration reference 50, as shown in FIG. 3b. The spectrophotometer 15 is re-translated to its normal online position subsequent to calibration, as shown in FIG. 3c. Translation of the spectrophotometer 15 between the measurement and calibration positions requires the use of a full-length translation system (not shown). The internal space required for accommodating such a system may result in an unacceptable enlargement of the printing device 10.
As an alternative to the automatic calibration arrangements described above, media for which spectral data has been obtained a priori (e.g., by performing offline measurements) may be manually fed through the printing device 10. The resulting spectral data generated by the spectrophotometer 15 may then be compared to the previously-obtained spectral data in order to determine the appropriate transform curve. This calibration technique, however, is time-consuming and requires a substantial amount of manual intervention.
Because use of the spectrophotometer 15 in the measurement and calibration modes is typically intermittent, it is generally desirable to automatically store the spectrophotometer 15 within the printing device 10 during periods of nonuse such that contamination of its optical surfaces is minimized. Storage of the spectrophotometer 15 in this manner within the limited internal space of a conventional printing device is problematic and may be exceedingly difficult in cases where a large portion of the available space is allocated to spectrophotometer 15 calibration features.
In view of the problems described above, there is a need for more efficient and effective systems and methods for calibrating and maintaining electro-optical instruments within printing devices.